Power allocation for a zoom lens is most important for a successful lens design. In this paper, six relations are derived to show that power allocation within the two groups of inverted telephoto vari-focal system will greatly depend on the system performances such as numerical aperture, field of view (FOV), back focal length (BFL), as well as the size of the lens volume. In addition, aberration correction also calls for a reasonable power allocation practical example is given to show the results.